Fingerprint sensor with liveness detection

ABSTRACT

A fingerprint sensor device with built-in liveness detection capabilities includes: an area sensor disposed on a top surface of a substrate; a stiffener disposed below a bottom surface of the substrate; a printed circuit making electrical connection to the sensor disposed below the stiffener; and a light source and a photodetector. At least one of the light source and photodetector is disposed on the printed circuit below the area sensor. The stiffener includes at least one through-hole located with respect to the light source or photodetector to allow light from the light source to transmit through the stiffener towards a finger located on the area sensor or to allow light reflected from the finger to pass through the stiffener to the photodetector.

CROSS-REFERENCE TO RELATED APPLICATIONS

The application is a continuation application of U.S. patent applicationSer. No. 15/748,943 filed on Jan. 30, 2018, which is a U.S. nationalstage application under 35 U.S.C. § 371 of International PatentApplication No. PCT/IB2017/000778 filed Jun. 2, 2017, both of which areincorporated herein by reference in their entireties.

TECHNICAL FIELD

The present invention relates to fingerprint sensors.

BACKGROUND

Biometrics can establish proof-of-identity and to some extent proof of auser's intent to enter into a given transaction. In practicalapplication, the usefulness of biometrics is limited by the precision ofthe biometric method (captured by false match and false non-match rates)and the quality of the system-level implementation.

One problem with biometric systems is that they can be spoofed, i.e.,tricked into accepting something other than the genuine biometric trait.For example, a face-recognition system may be spoofed using a photo. Andmost fingerprint sensors can be spoofed with fake fingers made fromdifferent materials including paper print-outs, rubber, gelatin,silicone, wood glue, etc., particularly when made electricallyconductive.

Fingerprint sensors employing the so-called “active thermal principle”are disclosed in U.S. Pat. Nos. 6,091,837 and 7,910,902, both to NgocMinh Dinh. The basic principle of the active thermal fingerprint sensoris the use of an array of PIN diodes as thermal sensors to differentiatethe ridges and valleys of the human fingerprint since the heat transferin these two areas are different. (A PIN diode is a diode with a wide,undoped intrinsic semiconductor region between a p-type semiconductorand an n-type semiconductor region. The p-type and n-type regions aretypically heavily doped to form ohmic contacts). A typical problem withthis kind of device is that latent prints left from a user on the sensormay be scanned and the sensor cannot determine when a real finger istouching the sensor. Liveness detection schemes, i.e., techniques fordetermining that a live subject is presenting a finger for fingerprintdetection, can be used to combat these spoofing techniques and problemswith latent prints.

Sensors are typically made by applying the sensing technology to asubstrate material. This deposit is then covered with a protectivecoating. The area of the substrate material surrounding the activesensing area needs to be covered to protect it from the environment(e.g., electro-static-discharge, moisture). Thus, separate livenessdetection sensors cannot be placed outside the sensor area.

There a several ways to characterize liveness detection techniques infingerprint sensors. One way to characterize these techniques is todistinguish in-band methods from methods requiring dedicated livenessdetection sensors. In-band methods look at the live image from thefingerprint sensor and try to distinguish features of live fingers whichare difficult to replicate in spoofing targets. Static in-band methodslook at features smaller than the ridge size, such as pores. Dynamicin-band methods look at how features of live images change over time:for example, the way a finger deforms when it lands on the sensor, orsweat escaping from the ridges as pressure increases. The advantage ofin-band methods is that they do not require dedicated hardware. Theirmain disadvantage is that they are limited by the sensor's spatial andtemporal resolution.

Hardware-based liveness detection methods require a dedicated sensor.There are three main methods known to the art. One known method is basedon blood oxygenation measurement through pulse oximetry. The methodrelies on differences in relative absorption between oxygenated andde-oxygenated hemoglobin: oxygenated hemoglobin absorbs more light inthe infrared spectrum while de-oxygenated hemoglobin absorbs more lightin the red spectrum. Typical blood oxygen monitors work with two LEDs,one with a peak wavelength of 660 nm (red) and one with a peakwavelength near 940 nm (infrared). The ratio of transmitted infrared tored light allows for an estimation of blood oxygenation.

Another technique is based on the so-called blanching effect. Thegeneral principle is that when the finger lands on the sensor, bloodrecedes with increasing pressure and the finger changes in color, i.e.,it gets lighter. This technique is described in Hengfoss, et al.,“Dynamic Liveness and Forgeries Detection of the Finger Surface on theBasis of Spectroscopy in the 400-1650 nm Region”, Forensic ScienceInternational 212 (2011) 61-68, the entirety of which is herebyincorporated by reference herein.

Another known technique is based on laser-doppler flowmetry. Thistechnique uses the Doppler shift effect to detect the movement of bloodparticles.

SUMMARY OF THE INVENTION

In embodiments, of a fingerprint sensor device with built-in livenessdetection capabilities, the fingerprint sensor device includes: an areasensor disposed on a top surface of a substrate; a stiffener disposedbelow a bottom surface of the substrate; a printed circuit makingelectrical connection to the sensor disposed below the stiffener; and alight source and a photodetector. At least one of the light source andphotodetector is disposed on the printed circuit below the area sensor.The stiffener includes at least one through-hole located with respect tothe light source or photodetector to allow light from the light sourceto transmit through the stiffener towards a finger located on the areasensor or to allow light reflected from the finger to pass through thestiffener to the photodetector.

In embodiments, the fingerprint area sensor device with built-inliveness detection capabilities, includes: an area sensor disposed on atop surface of a substrate, wherein the area sensor includes anintegrated pressure or proximity sensor; a stiffener disposed below abottom surface of the substrate; a flexible printed circuit makingelectrical connection to the sensor, the flexible printed circuitextending from the top surface of the substrate to a bottom side of thestiffener; and a light source and a photodetector disposed on theflexible printed circuit. The stiffener includes a first through-holelocated with respect to the light source to allow light from the lightsource to transmit through the stiffener towards a finger located on thearea sensor, and includes a second through-hole located with respect tothe photodetector to allow light reflected from the finger to passthrough the stiffener to the photodetector. A microcontroller isdisposed on the flexible printed circuit and configured to obtainreflected light data upon detection of a finger on the area sensorthrough the integrated pressure sensor or proximity sensor for use inliveness detection analysis.

In embodiments, a method includes the steps of detecting presence of afinger on a fingerprint area sensor using a first detection threshold;upon detecting the presence of the finger using the first detectionthreshold, perform liveness detection measurements using a light sourceand a photodetector disposed below a sensing area of the fingerprintarea sensor; detecting presence of the finger on the fingerprint areasensor using a second detection threshold greater than the firstdetection threshold; and upon detecting the presence of the finger usingthe second detection threshold, perform a fingerprint scan of thefinger.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate preferred embodiments of theinvention, as well as other information pertinent to the disclosure, inwhich:

FIG. 1 is a cross-sectional view a device having a fingerprint sensormodule mounted in a housing on a mounting bracket.

FIG. 2 shows the measured absorption spectrum in the 400 to 1000 nmrange for a fingerprint sensor module.

FIGS. 3 and 4 schematically illustrate side-by-side and stackedarrangements, respectively, of a light source.

FIGS. 5 to 5F are cross-sectional views of fingerprint sensorsconfigured for liveness detection.

FIGS. 6A and 6B illustrate embodiments of stiffeners with through-holesfor allowing transmitted light and reflected light to pass.

FIG. 7 is a flow diagram illustrating an embodiment of a combinedliveness detection and fingerprint scanning method for a sensor module.

FIG. 8 is a flow diagram illustrating an embodiment of a combinedliveness detection and fingerprint scanning method for an embeddedsensor module.

FIG. 9 shows a top view of an active area of a sensor.

FIG. 10 shows a cross-sectional view of another embodiment of afingerprint sensor configured for liveness detection.

DETAILED DESCRIPTION

This description of the exemplary embodiments is intended to be read inconnection with the accompanying drawings, which are to be consideredpart of the entire written description. In the description, relativeterms such as “lower,” “upper,” “horizontal,” “vertical,” “above,”“below,” “up,” “down,” “top” and “bottom” as well as derivative thereof(e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should beconstrued to refer to the orientation as then described or as shown inthe drawing under discussion. These relative terms are for convenienceof description and do not require that the apparatus be constructed oroperated in a particular orientation, nor to be in contact with eachother unless specified. Terms such as “overlap” refers to graphicallycover, but not necessarily in contact with each other. Terms concerningattachments, coupling and the like, such as “connected” and“interconnected,” refer to a relationship wherein structures are securedor attached to one another either directly or indirectly throughintervening structures, as well as both movable or rigid attachments orrelationships, unless expressly described otherwise. Likewise, termsconcerning electrical “connections” and “coupling” refer to arelationship wherein components communicate with one anotherelectrically either directly or indirectly through interveningstructures unless described otherwise.

In embodiments disclosed herein, a cost-effective, hardware-based, smallfootprint, dynamic liveness detection is realized below the sensorsubstrate. In embodiments disclosed herein, the liveness detectionscheme is designed for fingerprint sensors employing the active thermalprinciple, as described in, for example, U.S. Pat. Nos. 6,091,837 and7,910,902, both to Ngoc Minh Dinh, the entirety of which are herebyincorporated by reference herein.

The techniques described herein can be used with sensor modules that areconfigured to transmit an image to the host, or with embedded moduleswhere image processing, feature extraction and matching happens onmodule. In the case of sensor modules, liveness can be computed on thehost (for example, at the device driver level). In the case of embeddedmodules, liveness can be computed on the module's microcontroller, e.g.,an CORTEX® M4 processor from ARM running at 166 MHz.

FIG. 1 shows a cross-section of a device 10 having a fingerprint sensormodule 16 mounted in a housing 12 on a mounting bracket 14. The module16 can be, for example, the NB-2023-S2 (SPI interface) or NB-2023-U2(USB interface) fingerprint area sensor module available from NEXTBiometrics of Oslo, Norway. The mounting bracket 14 is coupled to thehousing 12 via screws or rivets 18. The housing 12 has an opening 22.The fingerprint sensor module 16 may include a bezel 20 that serves as afinger guide. Electronics 24, such as for addressing of the sensorarray, analog-to-digital conversion and/or signal processing, arecoupled to the fingerprint sensor module 16. A flexible printed circuit(not shown) is bonded to the top of the sensor 16 and wraps around theside of the sensor module 16 to make contact with the mounting bracket14, which provides a ground connection for the sensor module 16.

The fingerprint sensor module 16 includes a substrate layer, which istypically glass or polyethylene; a sensing layer, such as made oflow-temperature polysilicon; a protective coating layer; a stiffenerthat provides mechanical support for the sensing layer; and a signalprocessing layer, which may be a printed circuit board or flexibleprinted circuit with processing components thereon. These features areshown in more detail in FIG. 5 described below. The sensor technologyitself and the protective coating are not transparent but also notentirely opaque. The substrate material can be chosen to be transparent,e.g. glass. FIG. 2 shows the absorption spectrum in the 400 to 1000 nmrange for the NEXT Biometrics NB-S510-P2 sensor glass. The absorptionscale is base 10 logarithmic, i.e. a value of 1 corresponds to 10⁻¹transmission (10%); a value of 2 corresponds to 10⁻² transmission (1%),and so on.

In embodiments, one or more light sources (e.g., LEDs) and zero or morephotodetectors (e.g., photodiodes) may be placed underneath the sensorsurface. This arrangement could be for either measuring the blanchingeffect or for measuring blood oxygenation (in the latter case, at leasttwo LEDs are required). Side-by-side or stacked arrangements arepossible. FIG. 3 is a schematic illustration of an embodiment of aside-by-side arrangement 100 of an LED 110 and a photodiode 112 with aseparator 114 that blocks a direct light path between the two. FIG. 4shows a stacked arrangement 100A with the LED 110 a arranged above thephotodiode 112 such that there is not direct light path between the LED110 a and the photodiode 112. It should be understood that theorientation of the light source (LED) and photodetector (photodiode)could be reversed, with the photodiode above the LED, as long as thereis no direct light path from the LED to the photodiode.

It is preferred to block a direct light path between the light source(e.g., LED) and the photodetector (e.g., photodiode) so that lightreceived by the photodetector is light that was incident on the finger.This is shown in connection with FIG. 5. FIG. 5 shows a fingerprintsensor module 200 with integrated hardware based optical livenessdetection. The fingerprint sensor module 200 includes a sensor 202(which includes an array of sensor elements in an active area 205) and aprotective coating layer 204. The sensor layer 202 is formed on asubstrate layer 206. A stiffener 208 is provided below the substratelayer 204. In embodiments, the stiffener 208 can be a sheet or plate ofaluminum having a thickness in the range of, for example, 0.1 mm to 2mm, and in embodiments of at least 0.2 mm. A flexible printed circuit210 makes electrical connections between the fingerprint sensor 202 andelectronics 221. Additional electronic components 212 foranalog-to-digital conversion, sensor addressing and/or signal processingmay be placed on the flexible printed circuit. The flexible printedcircuit is used because the sensor contacts are on the finger-facingside of the module and the electronics are on the reverse side. Thestiffener 208 is used to protect sensor module from bending, whichoffers protection to components on the flexible printed circuit 210against mechanical damage from bending. One or more light sources 214,e.g., LEDs, and one or more photodetectors 216, e.g., photodiodes, aredisposed below the stiffener 208. In addition to providing the devicemechanical integrity, the stiffener 208 is configured to prevent adirect light path from light source(s) 214 to the photodetector(s) 216.In embodiments, one or more through-holes 218 are formed through thestiffener 208 above the light source(s) 214 to allow the lighttransmitted from the light source(s) 214 to pass through the otherwiseopaque stiffener 208 to the finger. One or more through-holes 219 arealso formed through the stiffener 208 to allow light reflected from thefinger to be received by the photodetector(s) 216.

It should be understood that while gaps are shown between the flexibleprinted circuit 210, the stiffener 208 and the substrate 206, this isonly for purposes of ease of illustration of these layers in theschematic illustration of FIG. 5. Any gaps between these layers shouldbe minimized to control reflections.

The distance between the transmitted light through-hole 218 and thereflected light through-hole 219 should be small, to maximize the amountof reflected light captured by the photodetector 216. In embodiments,the distance is in the range of 0.1 mm to 4 mm. In embodiments, thedistance is less than 0.25 mm. One way to achieve a small effectivedistance (shown as distance D on segment 208 a formed between thethrough-holes 218, 219) between the through holes 218, 219 withoutcompromising mechanical stability of the stiffener, is to shape one (orboth) through-holes 218, 219 as conical bores with the larger diameterfacing away from the light source 214 and photodetector 216, as shown inFIG. 5A. Segment 208 a serves as the separator shown in FIG. 3 thatblocks any direct light path between the light source and thephotodetector. In production, this can be achieved by placing twoconcentric bore-holes into the stiffener 208 from opposite directions.One of the bore-holes has a larger diameter and this bore hole does nottravel through the entire thickness of the stiffener, thus creating aconical shape in the resulting through-hole.

FIGS. 5B and 5C illustrate embodiments of fingerprint sensor modules200A and 200B, respectively. Like features are labeled with likereference numerals from FIG. 5. As shown in FIG. 5B, the flexibleprinted circuit 210A extends from the topside of the substrate 206,where it makes electrical connections to sensor 202 through a conductivelow temperature polysilicon area formed on the substrate 206 outside ofthe active area 205, around the side of the substrate 206 and stiffener208A to the bottom side of the stiffener 208A. Light source 214A,photodetector 216A and one or more additional electronic components 212(e.g., for analog-to-digital conversion and/or signal processing) are onthe side of the flexible printed circuit 210A facing away from thebottom side of the stiffener 208A. Through hole 220 is formed in theflexible printed circuit 210A to align with the through hole 218A in thestiffener 208A. Through-holes 220 and 218A allow light from light source214A to pass through the flexible printed circuit 210A and stiffener208A towards the finger, which is placed on the sensor 202. Through hole224 is formed in the flexible printed circuit 210A to align with thethrough hole 219A in the stiffener 209A. Through-holes 219A and 224allow light reflected from the finger to pass through the stiffener 208Aand the flexible printed circuit 210A to the photodetector 216A. Throughholes 220 and 224 in flexible printed circuit 210A are not needed if theflexible printed circuit 210A is sufficiently translucent such thatliveness detection is not impaired. Alternatively, as shown in FIG. 5C,the light source 214B and the photodetector 216B can be located on theside of the flexible printed circuit 210B that is facing the stiffener208B. In this embodiment, the light source 214B and photodetector 216Bare disposed at least partially within through holes 218B and 219B,respectively, within the stiffener 208B. In this way there is no issuewith respect to the flexible printed circuit 210B interfering with lightbeing transmitted from the light source 214B towards the finger andbeing received by the photodetector 216B from the finger, and thestiffener 208B blocks any direct light path between the light source214B and photodetector 216B.

In embodiments, the through-hole 218A and through-hole 219A can be onethrough-hole covering both through-hole 220 and through-hole 224,assuming the flexible circuit portion separating through-hole 220 andthrough-hole 224 provides sufficient blocking of light emitted from thelight source 214A from being received at the photodetector 216A.

In embodiments, the photodetector is placed below the light source(e.g., LED). For example, the LED and photodetectors are on differentsides of the flexible printed circuit, with their active sides facing inthe same direction. The component disposed on the face of the flexibleprinted circuit facing away from the finger would be reverse mounted. Inembodiments, the light source 214B of FIG. 5C can be used withphotodetector 216A of FIG. 5C. Alternatively, the light source can beplaced below the photodetector. That is, light source 214A of FIG. 5Bcan be used with photodetector 216B of FIG. 5C. The light source and thephotodetector can be laterally offset from one another or even alignedvertically with one another as long as the element on top does not blockeither the reflected light (in the case of the light source on top) orthe transmitted light (in the case of the light source on the bottom),as the case may be, so as to adversely affect the ability to performliveness detection analysis.

An embodiment of a light source/photodetector/stiffener configuration300 is shown in FIG. 6A. FIG. 6A shows both a top view (left) and across-section (right). The stiffener 308 has two relatively largerectangular slots or openings formed therein, with the first opening 310for allowing transmission of light from the light source 302 and thesecond opening 312 for passing reflected light to the photodetector 304.The shape of the sensor glass substrate is shown by dashed line 306. Ascan be seen in the cross-section, the light source 302 and photodetector304 are received in or otherwise positioned with respect to (i.e.,proximate) the openings 310, 312 so that the light path between the twois completely or substantially blocked by stiffener segment 308 a (i.e.,so that all or nearly all of the light received at the photodetector 304is reflected light from the finger and not light directly from the lightsource 302 to the photodetector 304). The configuration 300 maximizeslight emitted into the finger and light reflected from the finger andhas worked well in experimental set-ups. This configuration is moresusceptible, however, to ambient light being received at thephotodetector due to the size of the opening 312 in the stiffener 308.Ambient light is essentially noise that decreases the systemssignal-to-noise ratio (S/N).

FIG. 6B illustrates an alternative embodiment of a lightsource/photodetector/stiffener configuration 400. FIG. 6B shows both atop view (left) and a cross-section (right). The stiffener 408 has tworelatively small or narrow cylindrical slots or openings formed therein,with the first opening 410 for allowing transmission of light from thelight source 402 through the stiffener 408 and the second opening 412for allowing reflected light to pass through the stiffener 408 to thephotodetector 404. The shape of the sensor glass is shown by dashed line406. As can be seen in the cross-section, the light source 402 andphotodetector 404 are received in or otherwise positioned with respectto (i.e., proximate) the openings 410, 412 so that the direct light pathbetween the two is completely or substantially blocked by stiffenersegment 408 a (i.e., so that all or nearly all of the light received atthe photodetector 304 is reflected light from the finger and nottransmitted light from the light source 302). The configuration 400 isless sensitive to environmental ambient light, but the signal (lighttransmitted to the finger and reflected from the finger to thephotodiode) is also weaker. It should be appreciated that the conicalbore design of FIG. 5A can be used with the configurations of FIGS. 6Aand 6B.

The light source/photodetector/stiffener configuration should be setupto control environmental light. Both of the configurations of FIGS. 6Aand 6B prevent a direct light path between the light source and thephotodetector. But the configuration of FIG. 6B is less sensitive toenvironmental light. However, it is also less sensitive to the signal(light from the light source reflected by the finger). The configurationof FIG. 6A in the top half of the figure illuminates the finger moreevenly and allows more reflected light from the finger to reach thephotodetector. However, it also admits more environmental light. Inembodiments, one non-hardware approach to controlling for environmentallight involves taking one or more baseline measurements with the lightsource (LED) OFF, and to compare them with measurements where the LED isswitched ON. The difference between these measurements (or a differenceof averages) is the desired signal.

It should be understood that based on this disclosure one of ordinaryskill in the art can configure the shape of the holes in the stiffener,their location, size and spacing as well as the location relativethereto of the light source and photodetector so as to optimize for agiven design for transmission to the finger and reception from thefinger of light to the exclusion of ambient light, which is effectivelynoise. In embodiments, the shape and/or size of the stiffenerthrough-hole(s) that allows for transmission of light to the finger andthe shape of the stiffener through-hole(s) allow for reflected light topass to the photodetector are different. Efforts should be made toincrease the directionality of the light emitted from the light source,i.e., to provide a narrower, focused beam. If the beam's angle is toogreat, then too much light is dispersed within the substrate materialand not enough light will be reflected from the finger. Orientation ofthe light source and photodetector is also important. In an experimentalset up, a printed circuit board (PCB) with the light source was glued tothe sensor's glass substrate. With increasing finger pressure, thesubstrate and PCB were bent and the detector could be fooled by placingaluminum foil over the finger. It is thus important that the stiffeneris indeed “stiff”. Assuming an aluminum stiffener, a thickness of 1.5 mmproved sufficient. It is possible that thinner stiffeners will also workgiven different materials or stiffness and through-hole configuration.In embodiments, the minimum stiffness is a stiffness sufficient toprovide both mechanical protection for the device as described abovewhile at the same time providing protection against bending that causesnon-planarity (through bending) that might make the device subject tobeing fooled as described above. Different wavelengths for the lightsource were tried. An LED with peak wavelength of 570-580 nm provedbest. A broad spectrum (white) LED seemed to worked better than a narrowspectrum (green) LED

Experiments were performed based on liveness detection using theblanching effect. In the experiments, a temperature drift effect wasobserved, that could overlay the blanching effect. It was determined,therefore, that it is important to begin measurements early in thefinger-placing process. A high temporal measurement resolution may helpin distinguishing the temperature drift effect from the blanchingeffect; more than 250 Hz temporal resolution (i.e., number ofmeasurements per unit of time) is recommended.

A preferred embodiment makes use of the blanching effect. This solutionis preferred over blood oximetry and blood flow detection for tworeasons. First, it is low cost. LEDs and photodiodes in the visiblespectrum can be made from the cheaper Gallium phosphide (GaP), ratherthan the more expensive Gallium arsenide (GaAs) that is typically neededfor infrared LEDs and photodiodes. Infrared LEDs and detectors areneeded for blood oximetry. Surface mount device (SMD) laser elementsneeded for measuring the Doppler effect in blood-flow detection systemsare even more expensive. Second, the blanching effect technique is fast.Measuring the dynamics of the finger landing on the sensor takes only afew hundred milliseconds. In contrast, for reliably measuring bloodoxygenation, a few pulse cycles are needed (i.e., a few seconds).

As described above, one preferred embodiment places an LED and aphotodiode directly adjacent to each other. However, a direct light pathis prevented by a metal stiffener, placed directly in the light path. Inembodiments, the stiffener has conical burrows, with the larger radiijust touching (or nearly touching) each other at the top surface of thestiffener to maximize the amount of light reflected from the finger. Inembodiments, the light source(s) and the photodetector(s) are placed onthe flexible printed circuit within the fingerprint module. Thisflexible printed circuit also includes the signal processing elementsfor the fingerprint sensor. In embodiments, the same microcontrollerused for signal processing of fingerprints is used for controlling thelight source(s) and processing signals received by the photodetector(s)to do liveness detection. An analog to digital converter can beintegrated into the microcontroller for converting analog signals fromthe photodetector to digital information. In an alternative embodiment,a dedicated analog-to-digital converter can be used. In embodiments, bitdepth (i.e., number of bits available to quantify a given signal) is atleast 10 bits. The light may travel through the flexible printed circuit(see FIG. 5) if the flexible printed circuit is transparent enough.Alternatively, the flexible printed circuit could have through-holesthat align with those of the stiffener to allow for the light to travelfreely, as shown in FIG. 5B. Alternatively, the light source andphotodetector can be located on the stiffener facing side of the(flexible) printed circuit and aligned with through-holes in stiffener.

When the blanching effect principle is used, it would be advantageous inembodiments to correlate the pressure from a separate sensor (such as acapacitive sensor or piezoelectric sensor) built into the fingerprintsensor. The NEXT Biometrics fingerprint sensors identified herein have acapacitive proximity sensor (whose signal correlates to finger pressure)built-in, which could be used for the purpose. Liveness detectionmeasurement should happen within the first few hundred millisecondsafter the finger has begun to touch the sensor surface, as detected bythe capacitive sensor. The Next Biometrics scanners discussed abovetakes about 400 milliseconds to scan a fingerprint after detection of afinger via the capacitive force sensor, which is sufficient time toaccommodate liveness detection within the scan time. In embodiments, theliveness detection measurement begins when the finger touches the sensorand before the fingerprint scan begins. The fingerprint scan time andliveness detection time may overlap. With these design directives, asmall, but sufficiently robust blanching effect can be observed.

The signal from the photodiode is evaluated only when the signal from aproximity or pressure sensor is in a predetermined range, preferably thecapacitive sensor integrated into the fingerprint sensor.

In embodiments, a contact oil (e.g., silicone oil) with refractive indexlike that of the glass may be placed between light source and/or thephotodetector and the surface of the substrate to minimize unwantedreflection. In embodiments, the contact oil is placed between the bottomsurface of the substrate and the top surface of the stiffener.

It should be appreciated that blood oxygenation measurements could alsobe done through the sensor glass as a liveness detection mechanism,though it is anticipated that a longer time would be required (ascompared with the blanching effect technique) for the measurement.

In embodiments, two LEDs with peak wavelength 600 nm and 940 nm,respectively, could be used to measure blood oxygenation. Inembodiments, different photodetectors with different peak sensitivitiesand as little sensitivity overlap as possible would be provided tocollect the reflected light of different wavelengths.

In embodiments, a SMD laser light source could be used, to make use ofthe Doppler shift effect to detect blood flow.

In embodiments, the photodetector (e.g., a photodiode) is directlyintegrated into the sensing layer.

FIG. 9 shows a top view of an active area 900 of a sensor. As describedabove, the active area 900 includes an array 902 (arranged in rows andcolumns) of sensor elements 908. These elements are used for capturingfingerprint image data, e.g., using the active thermal principle. Acapacitive proximity sensor 904 is shown as a line going through thesensor area's center, parallel to the long edge. In embodiments, selectpixels of the sensing array 902 are replaced with photodetectors 910,i.e., be dedicated to photodetection rather than thermal sensing. Thesepixels are ideally concentrated in the area 906 near the sensor'scenter. FIG. 5D shows a cross-sectional view of an embodiment offingerprint sensor module 200C. This sensor module 200C includes asensor 202A with an active area 205A that includes one or morephotodetectors as described above in connection with FIG. 9. As such,the stiffener layer 208C only includes a through-hole 218B for passinglight transmitted from the light source 214B disposed on the flexibleprinted circuit 210C, which does not have a photodetector disposedthereon. Connections from the photodetectors in the area 205A can bemade to electronics 212 in the same manner as other connections from thesensor 202A.

In embodiments, a single LED could be used as both the light source andphotodetector, assuming sufficient sensitivity of the LED as aphotodetector. An example of this embodiment is shown in FIG. 5E, whichshows a cross-sectional view of an embodiment of fingerprint sensormodule 200D. In this embodiment, an element 230 on the flexible printedcircuit 212 serves as both light source and photodetector. Stiffener208D includes a through-hole 232 through which both transmitted andreflected light pass.

FIG. 5F is a cross-sectional view of another embodiment of fingerprintsensor module 200E. As with the embodiment of FIG. 5E, the stiffener208E includes a through-hole 234 through which both transmitted andreflected light pass. In this embodiment, light source 214B and 216B arelocated adjacent to one another on flexible printed circuit 208E andwithin or with respect to the same through-hole 234. In this embodiment,the light source 214B can be strobed at a high rate so as to avoidemitting interfering light during detection of reflected light by thephotodetector 216B. As described above, the light source andphotodetector do not need to be mounted on the same side of the flexibleprinted circuit.

In embodiments, a dedicated proximity or pressure sensor is used todetect finger placement on the sensor and trigger liveness detectionmeasurements.

Note that embodiments are describe above that use a flexible printedcircuit and a separate stiffener (e.g., a thin aluminum plate). Inembodiments, a rigid (non-flexible) printed circuit board assembly(PCBA) could serve the same dual function of printed circuit andstiffener. In this case, either vias or a separate flexible printedcircuit (FPC) or flexible flat cable (FFC) could be used to connect thePCBA and sensor. An example of this embodiment is shown in FIG. 10. FIG.10 shows a cross-sectional view of a fingerprint sensor module 1000. Asensor 1004 (which includes an array of sensor elements in an activearea 1006) and a protective coating layer 1008 are disposed oversubstrate 1002. The substrate is disposed on the top surface of PCBA1010. Electronics 1020, such as for addressing of the sensor array,analog-to-digital conversion and/or signal processing are disposed onthe bottom surface of the PCBA and coupled to the sensor module 1004 asdescribed above. Through-hole 1016 is formed through the PCBA 1010 toalign with light source 1012 disposed on the bottom surface of the PCBA,and through-hole 1018 is formed through the PCBA 1010 to align withphotodetector 1014 disposed on the bottom surface of the PCBA 1010. Ofcourse, as described above, embodiments where one of the light sourceand photodetector are disposed on the PCBA 1010 are contemplated as wellas embodiments where there is one through-hole in the PCBA 1010.

FIG. 7 illustrates an embodiment of a combined liveness detection andfingerprint scanning method for a sensor module that is interfaced to ahost device in order for the host device to do fingerprint comparisonsagainst stored fingerprint templates. Examples of such sensor modulesinclude the NEXT Biometrics NB-2023-S2 and NB-2023-U2 fingerprint areasensor modules.

At 702, the presence of a finger on the sensor is detected. This stepmay use capacitive finger-present detection methods an use a first(reduced) threshold.

At 704, assuming the first threshold is met or exceeded, a livenessdetection technique is begun. In this step the light source (LED(s)) isturned on.

At 706, dynamic liveness detection measurements are made with thephotodetector(s). Several measurements are made at different fingerpressures until the finger present detection at a second (increased)threshold, greater than the first threshold, (step 708) is made.

At 710, after the finger present detection at the second threshold, themeasured liveness detection data is transmitted to the host device foranalysis, i.e., for a determination if a “live” finger is present, e.g.,using the known blanching effect.

At 712, the fingerprint scan is commenced.

At 714, the scanned fingerprint image is transmitted to the host forfeature extraction and comparison with a stored template, for storage,or other use, using known techniques.

At 716, the method ends.

While FIG. 7 shows the liveness detection measurements ending with thedetection of the finger at the increased threshold (step 708), it shouldbe understood that in embodiments the liveness detection measurementscontinue and step 708 is used only as the trigger to begin thefingerprint scan and not to end the fingerprint measurements.

FIG. 8 illustrates an embodiment of a combined liveness detection andfingerprint scanning method for an embedded sensor module havingembedded fingerprint matching capabilities. Examples of such sensormodules include the NEXT Biometrics NB-1411-S and NB-1411-U fingerprintarea embedded sensor modules. The processor that handles the fingerprintmatching, or a different processor that is part of the module, isconfigured to perform liveness detection determinations.

At 802, the presence of a finger on the sensor is detected. This stepmay use capacitive finger-present detection use a first (reduced)threshold.

At 804, assuming the first threshold is met or exceeded, a livenessdetection technique is begun. In this step the light source (LED(s)) isturned on.

At 806, dynamic liveness detection measurements are made with thephotodetector(s). Several measurements are made at different fingerpressures until the finger present detection at a second (increased)threshold, greater than the first threshold, (step 808) is made.

At 810, after the finger present detection as the second threshold, aliveness detection determination is made without transmitting theliveness detection data to a host device for analysis. The determinationis based on, for example, the blanching effect.

In embodiments, at the same time the liveness detection result is beingcalculated, at 812, the fingerprint scan is commenced. In embodiments,the processor that handles image processing and matching (step 818) isidle during the scan operation, meaning it is free to handle livenessdetection processing before being taxed by the image processing andmatching operations.

At 814, it is determined if the liveness detection calculation indicateda live finger or not. If no live finger, then a failure (e.g., in theform of an authentication failure code) is sent to the host (step 816).At step 818, if the liveness detection indicated a live finger, thenfeature extraction and matching is performed. The result of step 818,e.g., either a positive authentication code or a negative authenticationcode, is transmitted to the host at step 820. The method ends at step822.

Although the invention has been described in terms of exemplaryembodiments, it is not limited thereto. Rather, the appended claimsshould be construed broadly to include other variants and embodiments ofthe invention that may be made by those skilled in the art withoutdeparting from the scope and range of equivalents of the invention.

What is claimed is:
 1. A fingerprint area sensor device with built-inliveness detection capabilities, comprising: a substrate having a topsurface and a bottom surface; an area sensor disposed on the top surfaceof the substrate, wherein the area sensor is configured forfinger-present detection; a printed circuit board disposed below thebottom surface of the substrate and electrically coupled to the areasensor; a light source configured to transmit light toward the areasensor; a photodetector configured to receive light reflected from thearea sensor; and a microcontroller in electrical communication with thearea sensor, the light source, and the photodetector, and wherein themicrocontroller is configured to obtain data for liveness detection upondetection of a finger on the area sensor using a first sensor threshold,and is further configured to initiate scanning of a fingerprint upondetection of the finger on the area sensor using a second sensorthreshold.
 2. The fingerprint area sensor device of claim 1, furthercomprising a stiffener between the printed circuit board and the areasensor.
 3. The fingerprint area sensor device of claim 2 wherein: thestiffener at least partially blocks a direct light path between thelight source and the photodetector; and the stiffener includes a firstthrough-hole at least partially aligned with the light source and asecond through-hole at least partially aligned with the photodetector,the first and second through-holes being adjacent to one another and asegment of the stiffener disposed between the first and secondthrough-holes blocks the direct light path.
 4. The fingerprint areasensor device of claim 1 wherein: the printed circuit board at leastpartially blocks a direct light path between the light source and thephotodetector; and the printed circuit board includes a firstthrough-hole at least partially aligned with the light source and asecond through-hole at least partially aligned with the photodetector,the first and second through-holes being adjacent to one another and asegment of the printed circuit board disposed between the first andsecond through-holes blocks the direct light path.
 5. The fingerprintarea sensor device of claim 1 wherein: the light source is between theprinted circuit board and the area sensor; the photodetector is betweenthe printed circuit board and the area sensor; and the fingerprint areasensor device further comprises a stiffener configured to block a directlight path between the light source and the photodetector.
 6. Thefingerprint area sensor device of claim 1, further comprising: astiffener between the substrate and the printed circuit board, thestiffener having a through-hole, wherein— the photodetector isintegrated with the area detector; and the light source is disposed inthe through-hole between the printed circuit board and the substrate. 7.The fingerprint area sensor device of claim 1, further comprising: astiffener between the substrate and the printed circuit board, thestiffener having a through-hole, wherein— the photodetector and thelight source are integrated into an element; and the element is disposedin the through-hole.
 8. The fingerprint area sensor device of claim 1wherein the area sensor includes an active area comprising a pluralityof sensor elements and wherein the photodetector is disposed in theactive area of the area sensor.
 9. The fingerprint area sensor device ofclaim 1 wherein the microcontroller is further configured to repeatedlyobtain data for liveness detection between the first sensor thresholdand the second sensor threshold.
 10. A fingerprint sensor device withbuilt-in liveness detection capabilities, comprising: a fingerprint areasensor comprising— an array of sensor elements configured for capturingfingerprint data; at least one photodetector disposed in the array; anintegrated sensor configured to detect when a finger is present on theactive area at a first sensor threshold and a second sensor threshold;and a light source configured to transmit light toward the fingerprintarea sensor; and a microcontroller in electrical communication with thefingerprint area sensor, the light source, and the photodetector, andwherein the microcontroller is configured to obtain data for livenessdetection upon detection of the finger on the fingerprint area sensorusing the first sensor threshold and initiate scanning of a fingerprintupon detection of the finger on the fingerprint area sensor using thesecond sensor threshold.
 11. The fingerprint sensor device of claim 10wherein the photodetector is one of a plurality of photodetectorsdisposed in the array in the active area.
 12. The fingerprint sensordevice of claim 11 wherein the plurality of photodetectors are disposedin a central area of the fingerprint area sensor.
 13. The fingerprintsensor device of claim 10 wherein the integrated sensor is a capacitiveproximity sensor.
 14. The fingerprint sensor device of claim 10 whereinthe integrated sensor is a pressure sensor.
 15. The fingerprint sensordevice of claim 10 wherein the plurality of sensor elements are activethermal sensing elements.
 16. A method for detecting spoofing attacks ona fingerprint sensor, the method comprising: detecting presence of afinger on a fingerprint area sensor using a first detection threshold;upon detecting the presence of the finger using the first detectionthreshold, performing at least two liveness detection measurements usinga light source and a photodetector disposed on or below a sensing areaof the fingerprint area sensor; detecting the presence of the finger onthe fingerprint area sensor using a second detection threshold greaterthan the first detection threshold; and upon detecting the presence ofthe finger using the second detection threshold, making a livenessdetection determination.
 17. The method of claim 16, further comprisingtransmitting a failure indication to a host upon detecting no livefinger.
 18. The method of claim 16, further comprising transmittingliveness detection measurement data to a host for liveness detectionanalysis.
 19. The method of claim 16, further comprising: upon detectingthe presence of the finger using the second detection threshold,performing a fingerprint scan of the finger while performing theliveness detection determination; and upon detecting a live finger,performing a fingerprint matching analysis.
 20. The method of claim 16wherein liveness detection measurements are taken at a rate of at least250 hertz (Hz).